The present invention relates generally to improvements in a hydraulic power brake system widely in use on automobiles and other vehicles, and more particularly to a dual type hydraulic circuit characterized by a control valve for regulating pressure levels of brake fluid in two independent hydraulic circuits of an automotive dual or split brake system.
Keeping pace with a recent trend in automobiles seeking higher driving speed under the progressively aggravating conditions of traffic environment, there has been and is presently a growing need for the safer and more reliable brake systems. To meet this need, most of the vehicles now in use are utilizing a dual or split type of hydraulic power brake system wherein a dual master cylinder is in communication with each of the wheel (brake) cylinders through two mutually independent piping systems thereby preventing a simultaneous failure of all wheel brakes when a fluid leak or any other pressure failure has occurred at a certain point in the hydraulic circuitry.
The typical method to serve this purpose has been to provide a so-called "diagonal" piping arrangement in which right front and left rear wheel cylinders are communicating within one piping system while left front and right rear wheel cylinders are connected by another piping system, or to provide a type piping arrangement which comprises a first piping system wherein the two front wheel cylinders are in communication with one of the rear wheel cylinders, and a second piping system wherein the two front wheel cylinders are connected to the other rear wheel cylinder. Such a dual type brake system commonly incorporates hydraulic pressure control valves (such as proportioning, load-sensing proportioning and inertia valves) in its circuits so as to improve directional control capability and steerability of the vehicle during brake applications. The reason for the provision of these control valves mentioned above is primarily for adjusting relative levels of hydraulic pressure between the front and rear wheel cylinders, especially while the pressure in a master cylinder or braking pressure is held at a higher level, in order to compensate for transfer of a considerable percentage of weight of the vehicle from the rear wheels to the front wheels when the brakes are applied for a sudden deceleration. In other words, the control valves are necessitated to attain, as much as practical, an optimum distribution of the hydraulic pressure to the front and the rear wheel cylinders; i.e., to adjust the front and rear brake forces to their optimum ratio.
In a dual type hydraulic circuit as employing the diagonal type piping arrangement, however, each one of the two piping systems has conventionally required one corresponding control valve and therefore a total of two control valves should be incorporated to achieve the optimum control of the brake forces between the front and rear wheel brakes. Unfortunately, the use of the two control valves contains a potential of a difference in hydraulic pressure level between the two piping systems because of a difference in performance characteristics between the two control valves (more particularly, due to a difference in preset value of load being applied to the two corresponding springs against which the control valves are operated). Another disadvantage incurred from using the two control valves is that a critical danger might happen when a pressure failure takes place by chance in either one of the two piping systems. That is to say, because the two control valves are designed and constructed so that the front and rear wheel brake pressures are controlled in accordance with an optimum distribution curve which is determined on the assumption that the both piping systems are always normal in function, a pressure failure in either piping system not only results in a drastic reduction of the entire braking effect but also may cause a large deviation from the predetermined optimum distribution curve between the front and rear brake pressure levels. In this condition, the front brake pressure tends to have a higher rising rate than the rear brake pressure, causing the front wheel to prematurely lock up and slip on the road surface.